وَيَقُولُ الَّذِينَ كَفَرُوا لَسْتَ مُرْسَلًا ۚ قُلْ كَفَىٰ بِاللَّهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ وَمَنْ عِنْدَهُ عِلْمُ الْكِتَابِ
Wayaqūlul-ladhīna kafarū lastma mursalan. Qul kafā billāhi shahīdan baynī wabaynakum wamin ‘indahu ‘ilmul-kitāb
And those who disbelieve say: You are not a messenger. Say: Allah is sufficient as a witness between me and you, and whoever has knowledge of the Book.
(Sūrat al-Ra‘d, No 13, Āyat 43)
According to Ibn Abbās bin ‘Abdul-Muttalib more than three hundred verses of the Quran refer to Ali bin Abu Talib ‘alayhis-salām. Ibn ‘Abbas says, ‘The phrase O you who believe was not revealed except that Ali was its leader, and its chief, and its most honorable reference’. The Quran reveals the great qualities of Imam Ali (a) in many of its verses, conveying the depths of nobility and piety that Imam possessed. The relationship between Imam Ali (a) and the Quran is undeniable. The Imam also talks about the Quran. He says: It is the Quran. If you ask it to speak it will not do so; but I will tell you about it. Know that it contains knowledge of what is to come about, stories of the past, cure for your ills and regulation for whatever faces you (Nahjul Balāgha, Sermon157,). He also says: No one will sit beside this Quran but that when he rises, he will achieve one addition or one diminution—addition in his guidance or elimination in his (spiritual) blindness. (Ibid, Sermon 175)
Introducing the thaql al-asghar (the smaller weighty thing – Ahlul Bayt) by the thaql al-akbar (the bigger weighty thing – the Quran), and explaining of the Quran by the Ahlul Bayt (a) is a clear manifestation of the Hadith al-Thaqalayn in which the Prophet sallal-lāhu ‘alayhi wa-ālihi wasallam announced that the two mighty things he was leaving behind would not separate from each other until they reach him at Kawthar.
Although Imam Ali (a) is not mentioned by name in the Quran, commentators from both the Ahl al- Sunnah and the Shī‘ah agree that many verses refer to his great status. Some of these verses clearly apply truly to Imam Ali and the Ahlul Bayt ‘alayhimus-salām, based on numerous commentators. Examples include the Āyat of Wilayah (Q 5:55), Āyat of Completion of Religion (Q 5:3), Āyat of Ulil-Amr (Q 4:59), Āyat of being with the Truthful ones (Q 9:119), Āyah of Qurbā (Q 42:23), Āyat of Tathīr (Q 33:33), Āyat of Mubāhila (Q 3:61), Āyat of Khayrul bariyya (Q 98:7), Āyat of the Night of Hijra (Q 2:207), Āyat of Disassociation from the polytheists (Q 9:1), and many others. The numerous mention of Imam’s qualities in the Quran is a sign of his high status in the eyes of Allah subhānahu wata‘ālā.
A verse that includes reference to Imam Ali (a) is the one mentioned at the beginning, the last verse of Sura Ra‘d. When the disbelievers reacted with doubt and skepticism to the Prophet (s) and his message, they made many demands. They asked for a different Quran, and for other miracles. The Prophet replies in this verse that even if they disbelieved, Allah and the one with the knowledge of Book were sufficient. There are many proofs of the miraculous nature of the Quran for those who use reason and are open to the truth.
There have been different interpretations as to who is the one with knowledge of the Book being referred to here. Tafsīr Majma ‘al-Bayan includes the following interpretations:
a. It refers to Allah himself, as He has knowledge of the Book. ‘Allamah Tabātabā’ī in al-Mīzān says this cannot be the case. The word Allah, denoting the complete essence of Allah, has already been mentioned. It would not be correct to then bring up a phrase representing His qualities for the same thing.
b. Believers from the Ahlul-Kitāb, who had knowledge of the Book. But this verse was revealed in Makkah, while the people of Ahlul-Kitāb accepted belief in Madinah.
c. Hadith from the Ma‘sūmīn (a) tell us that this refers to Imam Ali (a), and the rest of the Imams. Imam al-Sādiq (a) placed his hand on his chest and said: By Allah, the knowledge of Book, in its entirety, is with us. Many hadith also bear witness that no-one was more familiar and more knowledgeable about the Quran than Imam Ali (a).
Imam Ali (a) is undoubtedly the living, speaking, version of the guidance that is the Quran. He, and the Ahlul Bayt (a) after him, are the practical demonstrations of the theoretical principles outlined in the Quran. As you worship in the nights of Qadr and mourn the martyrdom of Imam Ali (a), remember the link between the two. They cannot be separated, and salvation is for the one who holds on fast to both.
Sources: Shaykh Tabarsī, Tafsīr Majma‘ al-Bayān; Āyatullāh Nāsir Makārim Shirāzī (Ed.), Tafsīr-e Namūneh; https://hawzah.net/fa/